Light olefins such as propylene are important organic chemical feedstocks. Commercial propylene is used to produce polypropylene, acrylonitrile and other chemicals. The propylene requirement is increasing year by year with a sharply increasing demand for polypropylene derivates. Worldwide annual propylene requirement amount increased from 15.2 million tons of twenty years ago to 51.2 million tons in 2000 and average annual increasing rate was 6.3%. It is estimated that there will be 86 million tons of propylene requirement in 2010 and average annual increasing rate will be 5.6% during that period.
Steam cracking and fluid catalytic cracking (FCC) are the main processes to produce propylene. Light oil such as naphtha can be used as the feedstock in steam cracking to produce ethylene and propylene, and the yield of propylene is only about 15% by weight. However, the feedstock of FCC process is heavy oil such as vacuum gas oil (VGO). Currently, 61% amount of propylene is from a byproduct of steam cracking process which produces ethylene, and 34% amount of propylene is produced by FCC, and about 5% amount of propylene is produced by propane dehydrogenation and ethylene-butene metathesis process.
Petrochemical industry will face to the shortage of light feedstock, deficiency of the processing capacity, high cost and other restrictive factors if ethylene and propylene are produced by the conventional petrochemical process such as steam cracking.
The role of FCC is increasingly concerned since FCC has advantage of wide adaptability of feedstock and flexible operation. Almost 50% amount of propylene is from FCC in U.S.A. The catalytic cracking technology with enhanced propylene yield is being improved quickly.
U.S. Pat. No. 4,980,053 discloses a process to produce gaseous olefins by catalytic conversion of hydrocarbons. Various fractions of petroleum, including residual oils and crude oils, are catalytically converted in fluidized or moving bed reactors with solid, acidic catalysts at a temperature of 500° C. to 650° C., and a pressure of 0.15 MPa to 0.3 MPa, with a weight hourly space velocity of 0.2 hr−1 to 20 hr−1 and catalyst to oil ratio of 2 to 12. Catalyst is continuously recycled between the reactor and the regenerator. The total yield of propylene and butene is about 40% by weight of the feedstock, and propylene yield is about 26.34% by weight of the feedstock.
WO00/31215A1 discloses a process for catalytic cracking to produce light olefin, which uses the catalyst comprising ZSM-5 and/or ZSM-11 as active component and much of inert substance as matrix. However, the yield of the propylene is less than 20% by weight of VGO.
U.S. Pat. No. 4,422,925 discloses a fluid catalytic cracking process in which a plurality cracking performance of hydrocarbon feedstocks including at least one gaseous paraffinic hydrocarbon feedstock and at least one liquid hydrocarbon feedstock are subjected to different cracking reaction conditions in several reaction zones in the present of regenerated catalyst to produce enhanced yield of light olefins.
Preserved amount of gasoline automobile has progressed rapidly due to economic development and the requirement of high quality gasoline is increasing. Presently improving gasoline octane number technologies include catalytic reforming, alkylation, isomerization, adding an gasoline octane number enhancing additive, and the like. The advantage of catalytic reforming is the heavy fraction of gasoline having high octane number, however, the octane number of light fraction gasoline is lower, in particular, the reforming catalyst cost is higher and the quality of feedstock is strictly required. Alkylation and isomerization technology can produce high octane number and good sensitivity gasoline which is a desirable high octane number clean gasoline component, there is, however, corrosion and environmental protection problem with the used catalyst. The octane number enhancing additive such as MTBE and ETBE can increase gasoline octane number and improve automobile performance, but the cost is higher. Catalytic cracking gasoline is a main source of automobile gasoline pool. The octane number of catalytic cracking gasoline is affected by lower octane number heavy gasoline fraction. Generally, the quality of FCC diesel is worse than straight-run diesel oil. FCC diesel oil is rich in mono ring aromatic hydrocarbons, which can be catalytically converted to high octane gasoline component and propylene product.
The prior arts are insufficient in paraffin molecules catalytic conversion, therefore the dry gas is increasing sharply when the propylene yield grows. Because of underutilization of the gasoline potential content of diesel oil which can produce high octane gasoline and light olefins, there still have room to improve for the quality and yield of gasoline. Therefore, it is desirable to develop a process for catalytic cracking of heavy feedstocks to produce an enhanced yield of high octane number gasoline and an enhanced yield of propylene to meet the market requirements of light olefin and gasoline.